1. Field of the Invention
The present invention relates in general to a system for generating atomized samples and injecting the atomized samples into analytical equipment, such as a plasma emission spectrometer, for example.
2. Description of the Background Art
Plasma emission spectrometers are highly sensitive devices that are employed to analyze samples for the presence of various elements or impurities therein. For example, drinking water is often analyzed for impurities by using an emission spectrometer. This is an important application due to increasingly strict impurity limits set for drinking water by the EPA. In particular, drinking water may contain trace levels of certain poisonous elements, such as lead and arsenic, but the allowable limits for these impurities are understandably very low. As a result, highly sensitive devices must be used to detect the levels of these impurities.
A plasma emission spectrometer operates by subjecting a sample to a high temperature plasma, which excites any elements that are present in the sample and causes them to emit radiation. The wavelengths of the radiation depend on the elements in the sample and thereby act as signatures for the elements. The spectrometer therefore separates the radiation into its individual wavelength components to facilitate identification of the specific elements in the sample. The intensity of the emissions at any given wavelength is proportional to the level of the corresponding elements in the sample.
In order to analyze a liquid sample, such as drinking water, with a plasma emission spectrometer, the sample must first be atomized with a carrier gas, such as argon, before being injected into the plasma. This is necessary because injection of a pure liquid into the plasma would result in extinguishment of the plasma, thereby preventing the device from functioning. To maximize detectability of the trace impurities in a liquid sample, it is therefore necessary to atomize the sample into very fine droplets by interaction of the sample with the high pressure carrier gas stream.
A device known as a nebulizer is employed to generate the atomized sample. A popular type of nebulizer known as a Babington pneumatic nebulizer operates by entraining a liquid sample in a gas stream in the following manner. The liquid sample is fed at low pressure through a first passage to a first outlet. The atomizing gas is fed at high pressure through a second passage out a second outlet that is parallel to and closely spaced to the first outlet. As the high pressure gas exits the second outlet, it creates a low pressure area around the second outlet that draws liquid exiting the first outlet into the gas stream. As the liquid is drawn into the gas stream, the droplets therein are broken down by the force of the high pressure gas into a fine aerosolized mist. If the mist is fine enough and can be injected into the plasma emission spectrometer at a low enough flow rate, e.g., less than 0.1 grams per minute, then the sample will not extinguish the plasma and can therefore be subjected to spectrum analysis.
One problem that is inherently associated with the use of nebulizers to generate emissions in a plasma spectrometer, is that the efficiency of such devices is fairly low, which limits the amount of detectable trace impurities that actually end up in the atomized sample. The atomization efficiency with which a nebulizer operates is measured as the percentage of the liquid sample that is actually atomized with the gas stream. Until recently, typical nebulizers had an efficiency of around 1 or 2%. In addition, it is often the case that only small samples of liquid are available for analysis. As a result, if a small sample of liquid having very low levels of trace impurities is atomized with a prior art nebulizer, the amount of detectable trace elements in the atomized sample would likely be too low to be detected by the spectrometer. In addition, small samples present the added problem of requiring very low flow rates into the nebulizer that leads to other problems, including for example, pulsation and intermittent aerosol formation.
Even if a higher efficiency nebulizer could be devised that operates well at low flow rates, another problem is presented when the sample is injected into a plasma emission spectrometer. In particular, the increased liquid content of the atomized sample has a tendency to reduce the temperature of the plasma, thus decreasing the intensity of the resulting emissions and counteracting the benefits of increase atomization efficiency. In some cases, the plasma may even be extinguished by the sample. One solution to this problem is to employ a cyclonic separator between the nebulizer and the spectrometer that separates excess liquid from the sample. Unfortunately, tests on such devices using the high efficiency nebulizer disclosed in the parent '456 application, indicate that the cyclonic separator has the effect of counteracting most of the increased efficiency of the high efficiency nebulizer. As a result, a need therefore remains not only for an increased efficiency nebulizer that can operate at low flow rates, but also for a sample injection system that can remove excess solvent from an atomized sample without counteracting the benefits of increased efficiency.